1. Field of Invention
The invention relates to new methods for the comminution of minerals to sub-micron particle sizes that are especially relevant to the processing of talc powders.
2. Description of Related Art
It is a well-known process to grind minerals in a ball mill in order to reduce the size of particles. This process, however, does not provide the ability to reduce the particle size of the majority of the particles below 2 microns equivalent spherical diameter. In order to produce particles with desirable properties, smaller particles are needed. In a modification to the traditional ball mill grinding method, U.S. Pat. No. 3,604,634 teaches a grinding method wherein an aqueous solution of at least 25 percent by weight of calcium carbonate is ground with a particulate grinding material long enough to dissipate at least 250 horsepower hours of energy per ton. According to the patent disclosure, sixteen hours of grinding using that process yielded a finished product with 97% of the particles smaller than 2 microns and 32% of the finished particles smaller than 500 nanometers.
Due to problems with spontaneous crystal dissolution-recrystallization in situations where the aqueous solution was overly saturated, U.S. Pat. No. 4,265,406 taught the addition of additives to the solution in order to reduce the particle size and thus increase the relative surface area of the powder.
In U.S. Pat. No. 4,325,514 (“the '514 patent”), comminution is referenced that can be performed either “wet or dry”. The method of comminution is via ball-milling. That specification, however, actually teaches away from the instant invention by noting that the preferred grinding method is an aqueous slurry as opposed to a dry mixture. The '514 patent claims a method of comminuting materials involving a rotating impeller being forced through an aqueous slurry containing the subject material in solution.
Various inventive steps have subsequently made upon the basic slurry grinding model; however, the focus was on dispersing the particles for better grinding on centrifuging them in order to obtain uniformity in size. See, for example, U.S. Pat. No. 4,793,985 to Price, et. al. and U.S. Pat. No. 4,845,191 to Hautier.
Virtually all of the aforementioned slurry grinding methods have the disadvantages of a large number of steps, complicated equipment and/or addition of additives. We have found a novel method for comminuting minerals to very small particle size that is especially useful in the comminution of talc powders.